The present application is based on Patent Application No. 2006-294218 being filed in Japan on Oct. 30, 2006, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a fixing roller being employed for a fixing device, which fixes an image on a transfer paper by heating and pressurizing, in an image forming apparatus such as a copier, a printer, a facsimile, and the like that employ a electrophotographic system; and also relates to a fixing device and an image forming apparatus that are provided with this fixing roller.
2. Description of the Related Art
In a electrophotographic image forming apparatus, at least one of a pair of fixing rollers forming a nip houses a heat source therein so as to serve as a fixing roller (a heating roller), and by inserting a piece of paper, holding a toner image that has not been fixed, between the nip portion of the pair of rollers, the toner is fixed on the paper, which is referred to as a heating roller fixing method and is widely employed. In such a heating roller fixing method as has been described hereinabove, since non-adhesion to the toner is strongly required for the fixing roller, a fluorine resin layer of non-conductivity, which is excellent in release properties, is often formed on the surface of the fixing roller by coating or tube coating.
However, since the fixing roller is rubbed by a recording medium or a pressure roller, there are such problems a follows: When the surface of a fixing roller is formed with a fluorine resin layer of non-conductivity, which is, to be specific, a pure fluorine resin layer having insulation properties, the fluorine resin layer is charged significantly negatively by frictional electrification; and when the toner has a positive polarity, electrostatic offset easily occurs. In addition, a pure fluorine resin layer has a difficulty in mechanical strength, so that from a viewpoint of wear resistance, it was difficult to achieve a longer life of the fixing rollers.
Therefore, it was disclosed that the electrostatic offset is improved by supplying conductivity to the surfaces of the fixing rollers. For example, the Patent Reference 1 discloses a method to restrain the frictional electrification of a fixing roller by dispersing conductive carbon black across the fluorine resin layer which serves as a release layer. However, in accordance with the method being disclosed in the Patent Reference 1, addition of the carbon black deteriorates the smoothness of the surface of the fluorine resin layer, whereby the release property of the recording medium is reduced. In addition, when a wear resistant member having insulation property or conductivity is added in order to enhance the wear resistance, the surface of the fluorine resin layer is subject to the deterioration of the smoothness and the release property.
In addition, the Patent Reference 2 discloses a fixing roller that restrains the electrostatic offset from occurring, by roughening the surface of a conductive cylindrical cored bar so as to be corrugated, and coating it with a non-conductive fluorine resin layer, so as to form a conduction pathway between the tip portions of the convex portions of the corrugation of the cored bar surface and the surface of the non-conductive fluorine resin layer, by utilizing an insulation breakdown. However, even the method being disclosed in the Patent Reference 2 has such a problem as a part, in which the tip of the convex portion of the cored bar is exposed from the surface of the non-conductive fluorine resin layer, has an inferior release property, so that the toner, paper powders, and the like are easy to attach to. In addition, since the volume resistivity of the roller swings toward the low resistance, depending on the ratio that the exposed portion of the cored bar accounts for in the entire surface of the roller, there is a concern that a transfer electric current being applied to the transfer roller will flow to the fixing roller by way of the recording medium, which is referred as a leakage of the transfer electric current, whereby the fixing roller will be charged, so as to generate an electrostatic offset.